I. Optical Amplifiers
The basic elements of a communication system are a transmitter, a receiver, and a transmission medium. Optical fibers are today the transmission medium of choice for sending voice, video, and data signals over long distances. Although modern fibers have very low losses per unit length, long fiber spans, e.g., cables extending from one city to another, require periodic amplification of the transmitted signal to ensure accurate reception at the receiver.
Erbium doped fiber amplifiers have been developed to satisfy this need for signal amplification. Such amplifiers consist of a length of optical waveguide fiber, e.g., 5 to 30 meters of fiber, which has been doped with erbium. The quantum mechanical structure of erbium ions in a glass matrix allows for stimulated emission in the .about.1500 to .about.1600 nanometer range, which is one of the ranges in which optical waveguide fibers composed of silica exhibit low loss. As a result of such stimulated emission, a weak input signal can achieve more than a hundred fold amplification as it passes through a fiber amplifier.
To achieve such stimulated emission, the erbium ions must be pumped into an excited electronic state. Such pumping can take place in various pump bands, the most effective of which include those having midpoint wavelengths of .about.980 nanometers and .about.1480 nanometers. Efficient semiconductor laser sources are available for both of these pump bands. As would be expected, trade-offs exist between these pump bands, with the 980 band providing lower noise in the amplified signal and the 1480 band providing a lower propagation loss for the pump light, which is of value when remote pumping is to be performed.
Although stimulated emission occurs throughout the 1500 to 1600 nanometer range, the amount of amplification achieved is not uniform throughout this range. These variations in gain produce problems in multiplexed systems where a group of wavelengths are used to simultaneously transmit multiple signals down an optical fiber. Such multiplexing is of great commercial value since it allows significantly increased transmission capacity per fiber. Indeed, a current priority in the telecommunications industry is to upgrade existing one wavelength transmission systems to a multi-wavelength environment in a cost effective manner so as to address the ever increasing demand for greater signal carrying capacity.
In a typical application, a multi-wavelength signal carried on an optical fiber will be subjected to repeated rounds of amplification as it passes from the transmitter to the receiver. At each such stage, any differences in amplification which may exist at the various wavelengths will compound, with the wavelengths subject to more amplification becoming ever stronger at the expense of those subject to less amplification. Various approaches have been used in the art to address this non-uniform amplification problem.
One of the most basic approaches involves the selection of the wavelengths used to transmit the multiple signals. As is well known in the art, the gain spectrum of an erbium doped fiber amplifier is flatter in the "red band," i.e., in the longer wavelength region from about 1540-1545 nanometers to about 1565 nanometers, than in the "blue band," i.e., in the shorter wavelength region from about 1525 nanometers to about 1535-1540 nanometers. In particular, a very flat gain in the red band can be achieved by adjusting the fraction of erbium ions in the excited ("inverted") state through the selection of the length of the fiber amplifier and the level of pumping applied to the fiber.
To take advantage of this flatness, wavelength multiplexed systems employing erbium doped fiber amplifiers have had their signal channels in the red band. In addition, to address residual non-uniform gain, the signal input powers at the transmitter have been adjusted to take account in advance of the differential amplification which will occur as the signal is repeatedly amplified during its passage to the receiver.
To expand the useable wavelength range provided by erbium doped fiber amplifiers into the blue band, filters have been proposed to flatten the amplifier's gain spectrum. The standard assumption which is made in designing a practical filter for this purpose is that the gain of the amplifier is essentially "homogeneous" in character, i.e., that the gain can be described by the homogeneous model discussed in, for example, C R Giles, et al., "Modeling erbium-doped fiber amplifiers", J Lightwave Tech, vol. 9, pp. 271-283, 1991, and C R Giles, et al., "Optical amplifiers transform long-distance lightwave telecommunications", Proc IEEE, vol. 84, pp. 870-883, 1996. The essence of this assumption is that the gain of an amplifier is determined by the average inversion of the active species, e.g., the erbium ions in an erbium doped fiber amplifier, irrespective of the particular signal wavelengths, signal powers, pump wavelength, and pump power which produced that average inversion. Looked at another way, the assumption of homogeneous broadening means that if the gain at any one wavelength is by some means stabilized to a particular value then a gain at the other wavelengths is similarly stabilized (the stabilized value of the gain being different at different wavelengths).
By means of this assumption, a gain spectrum for an amplifier is calculated for a given average inversion and that gain spectrum is used to design a filter which can flatten the spectrum. A set of signal wavelengths when applied to the amplifier will then see a flattened gain spectrum provided that the average inversion in the presence of those signal wavelengths is the average inversion used in the design of the filter. The degree of flattening will, of course, depend on how well a manufactured filter actually has the desired attenuation spectrum.
Rather than calculating the gain spectrum using the homogeneous model, one could, for example, measure the gain spectrum of an actual amplifier and use that measured gain spectrum to design the filter. This empirical approach, however, also implicitly adopts the homogeneous model in that it is assumed that the gain spectrum will be flattened for any set of signal wavelengths and powers within the amplifier's operating range that has the same average inversion as that which existed when the empirical gain spectrum was measured.
The above approaches for implementing a gain flattening filter work well for signal wavelengths in the red band. Surprisingly, in accordance with the invention, it has been discovered that the homogeneous model does not work well in the blue band. Rather, this band exhibits substantial inhomogeneous behavior. Specifically, when at least one signal wavelength is in this band, the gain spectrum can no longer be described by a single average inversion which applies to all active species. This inhomogeneity leads to a variety of important consequences in connection with the design, implementation, and use of optical amplifiers, components of such amplifiers, and systems employing such amplifiers.
Inhomogeneous line broadening in erbium doped fiber amplifiers (EDFAs), in particular, "hole burning," has been reported in the literature. As described therein, this effect has a line width (an affected portion of the spectrum) of about 10 nm. The effect is dependent upon the degree of saturation of the amplifier and is always centered at the saturating signal wavelength. Discussions of the hole burning effect can be found in M Tachibana, et al., "Gain cross saturation and spectral hole burning in wideband erbium-doped fiber amplifiers", Opt Lett, vol. 16, pp. 1499-1501, 1991; H Chou, et al., "Inhomogeneous gain saturation of erbium-doped fiber amplifiers", in Proc. Optical Amplifiers and their Application, Davos, Switzerland, 1995, pp. 92-95; and A K Srivastava, et al., "Room temperature spectral hole-burning in erbium-doped fiber amplifiers", in Proc. Optical Fiber Communication Conference, San Jose, Calif., 1996, (Tu G7), pp. 33-34. For other discussions of inhomogeneous effects in EDFAs see E Desurvire, et al., "Gain hole-burning at 1.53 .mu.m in erbium-doped fiber amplifiers", IEEE Phot Tech Let, vol. 2, pp. 246-248, 1990; J L Zyskind, et al., "Determination of homogenous linewidth by spectral gain hole-burning in an erbium-doped fiber amplifier with GeO.sub.2 :SiO.sub.2 core", IEEE Phot Tech Let, vol. 2, pp. 869-871, 1990; and E Desurvire, et al., "Study of spectral dependence of gain saturation and effect of inhomogeneous broadening in erbium-doped aluminosilicate fiber amplifiers", IEEE Phot Tech Let, vol. 2, pp. 653-655, 1990.
Significantly, the reported inhomogeneous effects have been small, e.g., at most one dB. In contrast, in accordance with the invention, multi-dB distortions in the gain spectrum of EDFAs have been observed as a result of having at least one signal wavelength in the blue band.
II. Gain Tilt in WDM Transmission Systems
In wavelength division multiplexed (WDM) transmission systems that employ known optically pumped optical amplifiers in their transmission paths the phenomenon of gain tilt presents problems. Under different operating conditions, the amplifier amplifies the different channels to different relative extents such that any passive system that is designed to equalize the power output of the channels for one specific set of operating conditions is liable to fail to provide equalization when those conditions are changed. One of the aspects of the invention involves minimizing the adverse effects of this gain tilt phenomenon.